UNIVERSITY  OF  CALIFORNIA  PUBLICATIONS 

COLLEGE  OF  AGRICULTURE 

AGRICULTURAL  EXPERIMENT  STATION 
BERKELEY,  CALIFORNIA 


PRUNE  GROWING  IN  CALIFORNIA 


BY 

A.  H.  HENDRICKSON 


BULLETIN  No.  328 

June,  1921 


UNIVERSITY  OF  CALIFORNIA  PRESS 
BERKELEY,  CALIFORNIA 


David  P.  Barrows,  President  of  the  University. 

EXPERIMENT  STATION  STAFF 

HEADS  OP  DIVISIONS 

Thomas  Forsyth  Hunt,  Dean. 

Edward  J.  Wickson,  Horticulture  (Emeritus). 

Walter  Mulford,  Forestry,  Director  of  Resident  Instruction. 

C.  M.  Haring,  Veterinary  Science,  Director  of  Agricultural  Experiment  Station. 

B.  H.  Crocheron,  Director  of  Agricultural  Extension. 
Hubert  E.  Van  Norman,  Vice-Director;  Dairy  Management. 

James  T.  Barrett,  Acting  Director  of  Citrus  Experiment  Station ;  Plant  Pathology. 
William  A.  Setchell,  Botany. 
Myer  E.  Jaffa,  Nutrition. 
Ralph  E.  Smith,  Plant  Pathology. 
John  W.  Gilmore,  Agronomy. 
Charles  F.  Shaw,  Soil  Technology. 

John  W.  Gregg,  Landscape  Gardening  and  Floriculture. 
Frederic  T.  Bioletti,  Viticulture  and  Fruit  Products. 
Warren  T.  Clarke,  Agricultural  Extension. 
John  S.  Burd,  Agricultural  Chemistry. 
Charles  B.  Lipman,  Soil  Chemistry  and  Bacteriology. 
Ernest  B.  Babcock,  Genetics. 
Gordon  H.  True,  Animal  Husbandry. 
Fritz  W.  Woll,  Animal  Nutrition. 
W.  P.  Kelley,  Agricultural  Chemistry. 
H.  J.  Quayle,  Entomology. 
Elwood  Mead,  Rural  Institutions. 
H.  S.  Reed,  Plant  Physiology. 
L.  D.  Batchelor,  Orchard  Management. 
J.  C.  Whitten,  Pomology. 
fFRANK  Adams,  Irrigation  Investigations. 

C.  L.  Roadhouse,  Dairy  Industry. 

F.  L.  Griffin,  Agricultural  Education. 
John  E.  Dougherty,  Poultry  Husbandry. 
W.  B.  Herms,  Entomology  and  Parasitology. 
L.  J.  Fletcher,  Agricultural  Engineering. 
Edwin  C.  Voorhies,  Assistant  to  the  Dean. 


DIVISION  OF  POMOLOGY 

J.  C.  Whitten  F.  W.  Allen  L.  C.  Barnard 

W.L.Howard  G.  L.  Philp*  C.L.Austin 

W.  P.  Tufts  J.  P.  Bennett  L.  H.  Day 

E.  L.  Overholser  F.  E.  Neer  E.  M.  Russ 

A.  H.  Hendrickson  W.  P.  Duruz 


f  In  cooperation  with  Office  of  Public  Roads  and  Rural  Engineering,  U.  S.  Department  of 
Agriculture. 


PRUNE  GROWING  IN  CALIFORNIA 

By  A.  H.  HENDRICKSON 


Prunes  are  grown  in  nearly  every  part  of  California.  Some  of  the 
most  important  considerations  affecting  the  choice  of  a  location  are 
given  in  this  circular.  The  oldest  and  most  important  prune  sections 
are  located  in  the  central  coast  counties.  Increased  acreages  of  this 
fruit  are  yearly  being  brought  into  bearing  in  the  central  districts  and 
in  the  great  interior  valleys.  In  the  mountain  districts,  immediately 
upon  the  coast,  and  in  the  extreme  southern  part  of  the  state,  however, 
the  commercial  production  of  this  crop  is  not  of  much  importance. 

Prunes  are  grown  on  a  variety  of  soils.  The  chief  requirements 
are  that  the  soil  be  reasonably  fertile,  easily  worked,  and  preferably 
at  least  six  or  eight  feet  deep.  On  the  fertile  valley  soils  the  trees 
eventually  grow  to  larger  size  than  on  the  shallow  soils  often  found 
in  the  foothills.  Hardpan  closer  than  six  feet  from  the  surface  is  often 
indicated  by  stunted  trees  which  have  a  tendency  to  mature  their 
leaves  and  fruit  comparatively  early. 

An  unfailing  source  of  water  for  irrigation  is  of  utmost  importance. 
Formerly  prunes  were  grown  in  many  districts  without  irrigation,  but 
this  practice  is  being  rapidly  superseded  by  a  cultural  system  that 
includes  at  least  one  or  two  waterings  during  the  growing  season.  The 
necessity  of  water  during  this  season  was  clearly  shown  in  the  dry 
years  of  1919  and  1920.  Without  water  during  the  growing  season, 
the  crop  runs  to  small  sizes,  particularly  when  the  set  is  heavy. 

The  prune  requiresa  long  season  of  clear  warm  weather  for  proper 
maturity.  Foggy  regions  adjacent  to  the  ocean  or  to  San  Francisco 
Bay  are  not,  as  a  rule,  suited  to  its  needs.  The  location  should  be  free 
from  late  spring  frosts  and  early  fall  rains.  Local  observation  by  old 
residents  is  often  of  great  value  in  considering  these  factors.  Late 
spring  frosts  may  seriously  reduce  the  crop,  although  this  danger  is 
not  so  great  with  prunes  as  with  other  tree  fruits  which  blossom  early. 
Early  fall  rains  interfere  with  the  picking  and  drying  operations. 
Evaporators  are  now  coming  into  use  where  fall  rains  may  be  expected. 
In  some  sections  the  growers  are  using  evaporators  to  cure  the  entire 
crop. 


UNIVERSITY   OF    CALIFORNIA EXPERIMENT    STATION 


CHOICE  OF  NUESEEY  STOCK 

Fully  mature,  one-year-old  trees  are  best  for  general  planting. 
They  should  be  free  from  insects  and  disease.  Trees  affected  with 
crown  gall,  indicated  by  warty  swellings  on  the  roots,  should  not  be 
planted  under  any  circumstances.  Data  secured  at  the  University 
Farm  by  the  Division  of  Pomology  show  that  the  medium  sized  tree 
calipering  one-half  to  five-eighths  of  an  inch  just  above  the  union  is 
the  ideal  deciduous  fruit  tree  to  plant.1  Smaller  trees,  if  well  grown 
and  not  diseased,  may  also  be  planted  with  success.  Larger  trees 
which  measure  an  inch  or  more  in  diameter  at  the  crown,  with  a  corre- 
sponding development  of  lateral  branches,  do  not  make  so  satisfactory 
trees  the  first  season  as  do  the  smaller  sizes.  Later  observations  showed 
that  in  many  cases  the  smaller  tree  had  actually  surpassed  the  larger 
ones  in  diameter  at  the  end  of  the  second  season. 

Prunes  are  grown  principally  on  three  kinds  of  root  stocks:  the 
myrobalan,  the  almond,  and  the  peach.  As  a  result  of  an  inquiry 
made  by  the  Division  of  Pomology  in  1919  among  the  nurserymen  of 
the  state,  the  popularity  of  these  stocks  was  found  to  be  as  follows, 
indicated  by  the  percentage  of  trees  propagated  on  each :  63.3  per  cent 
were  grown  on  myrobalan ;  10.4  per  cent  on  almond ;  21.2  per  cent  on 
peach.    In  addition,  5.1  per  cent  were  propagated  on  apricot. 

The  myrobalan  stock  is  preferred  for  deep,  moist  soils,  such  as 
generally  occur  on  the  valley  floors.  It  has  a  wide  range  of  adaptation 
and  is  successfully  used  on  many  different  soil  types.  This  stock  is 
also  considered  to  be  adapted  to  locations  where,  at  times,  there  may 
be  too  much  water.  The  peach  and  almond  stocks,  as  a  rule,  are  con- 
sidered best  for  the  drier,  gravelly  soils.  Peach  stock  is  often  pre- 
ferred in  the  shallow  foothill  soils  and  in  some  of  the  sandy  loams 
where  the  peach  is  known  to  do  well.  Either  peach  or  almond  is  more 
susceptible  than  myrobalan  to  injury  from  extremes  of  soil  moisture. 

The  common  varieties  of  prunes  now  being  grown,  with  few  ex- 
ceptions, make  strong  unions  with  the  foregoing  stocks.  Reports  indi- 
cate that  the  Robe  de  Sergeant  does  not  show  a  strong  affinity  for  either 
the  peach  or  almond  root.  Furthermore,  certain  varieties  of  prunes 
arc  apt  to  make  a  poor  union  with  apricot  root  stocks. 


i  Monthly  Bull.  State  Com.  Hort.,  Vol.  7,  No.  4,  April,  1918. 


Bulletin  328  PRUNE  GROWING  IN  CALIFORNIA 


PLANTING  AND  CARE  OF  YOUNG  ORCHARDS 

The  ultimate  necessity  of  irrigating  the  prune  orchard  should  be 
carefully  considered  before  planting.  Young  prune  orchards  in  many 
districts  are  often  grown  successfully  without  irrigation  for  the  first 
four  or  five  years.  For  the  bearing  orchard,  however,  the  long  dry 
California  summers  make  irrigation  imperative  in  most  sections.  The 
orchard  should,  by  all  means,  be  prepared  for  irrigation  before  the 
trees  are  planted.  The  alfalfa  grower  does  not  wait  until  after  the 
crop  is  planted  before  leveling  his  land.  Why  should  the  prune  grower 
not  prepare  his  land  before  planting?  Water  does  not  run  uphill  in 
a  prune  orchard  any  more  than  in  an  alfalfa  field.  Proper  prepar- 
ation before  planting  means  a  great  saving  in  the  end  in  money  and 
labor. 

Prunes  are  usually  planted  on  the  square  system,  from  22  to  25 
feet  apart.  The  shorter  distance  is  used  where  the  soil  is  shallow  or 
of  a  sandy  nature.  The  size  of  mature  trees  under  similar  conditions 
affords  the  grower  an  indication  as  to  whether  or  not  this  distance  is 
right.  On  the  deep,  fertile  soils,  25  feet  should  be  the  minimum.  Some 
growers  prefer  27  or  even  30  feet  under  the  latter  conditions.  With 
the  hexagonal  system,  where  each  tree  is  equally  distant  from  all  im- 
mediately surrounding  trees,  about  15  per  cent  more  trees  may  be 
planted  than  with  the  square  system.  The  following  table  gives  the 
approximate  number  of  trees  per  acre  for  the  square  and  hexagonal 
systems : 

TABLE  OF  PLANTING  DISTANCES 

Hexagonal 
124 
104 

87 

80 

74 

64 

55 

Trees  should  be  planted  as  soon  as  the  winter  rains  have  moistened 
the  soil  to  a  depth  of  2  or  3  feet.  January  and  February  are  the  best 
months.  Planting  may  be  done  as  soon  as  the  trees  are  received  from 
the  nursery  and  the  ground  is  in  suitable  condition.  Prunes  have 
been  planted  successfully  as  late  as  March  or  even  the  first  of  April, 
but  such  late  planting  is  not  advisable  if  it  is  possible  to  do  the  work 
earlier.  It  is  important  to  have  the  young  tree  established  and  capable 
of  supplying  water  to  the  leaves  as  soon  as  the  latter  begin  to  develop. 


Number  of  Trees 

Distance 

per  Acre 

Apart 

Square 

20  X  20 

108 

22  X  22 

90 

24  X  24 

76 

25  X  25 

70 

26  X  26 

64 

28  X  28 

56 

30  X  30 

48 

6  UNIVERSITY   OF    CALIFORNIA — EXPERIMENT   STATION 

Often  late  planted  trees  make  a  short  growth  which  later  withers  and 
dies.  This  growth  is  made  at  the  expense  of  the  stored  food  reserves 
in  the  buds  and  twigs.  When  this  reserve  is  exhausted  no  further 
growth  can  be  made  unless  the  roots  have  become  established  and  are 
able  to  replenish  the  supply. 

As  soon  as  the  trees  are  received  from  the  nursery  they  should  be 
removed  from  the  bale  and  "heeled-in"  in  a  sandy,  well  drained  spot 
convenient  to  the  orchard.  A  trench  12  to  14  inches  deep  and  about 
16  inches  wide  is  suitable  for  this  purpose.  One  side  is  left  sloping 
at  a  slight  angle  to  the  perpendicular.  The  trees  are  spread  out  and 
laid  against  this  sloping  side  and  loose  soil  is  worked  down  between 
the  roots.  The  entire  root  system  is  then  covered  to  a  depth  sufficient 
to  prevent  drying. 

In  planting,  the  hole  is  dug  large  and  deep  enough  to  hold  the  roots 
conveniently.  The  tree  is  usually  planted  so  that  it  will  stand  at  about 
the  same  depth  at  which  it  grew  in  the  nursery.  Many  trees  have 
been  injured  by  too  deep  planting,  especially  where  the  soil  is  heavy. 
If  the  soil  is  light  and  there  is  danger  of  the  wind  blowing  the  soil 
from  the  roots,  the  tree  should  be  planted  slightly  deeper.  The  soil 
should  be  worked  between  the  roots  and  firmly  tramped.  Success  in 
planting  depends  largely  on  setting  the  trees  firmly.  The  top  two  or 
three  inches  of  soil  should  be  left  loose.  The  use  of  dynamite  is  un- 
necessary, except,  possibly,  where  there  is  an  impervious  layer  a  few 
feet  below  the  surface.  After  setting  and  straightening  the  young 
trees  they  should,  if  straight  whips,  be  cut  off  24  to  30  inches  above 
the  ground.  Whitewash  applied  immediately  after  setting  is  a  valu- 
able measure  against  sunburn. 

A  convenient  method  of  planting  trees  to  secure  straight  rows  is 
by  means  of  a  planting  board.  The  planting  board  is  simply  a  piece 
of  scantling  about  five  feet  long  and  four  inches  wide.  Notches  two 
inches  deep  are  cut  in  the  middle  of  one  side  and  in  the  middle  of  each 
of  the  ends.  The  apexes  of  these  notches  then  lie  in  a  straight  line 
bisecting  the  board  from  end  to  end.  The  field  is  staked,  placing  a 
stake  for  each  tree  to  be  planted.  The  planting  board  is  then  placed 
with  the  notch  in  the  side  held  snugly  against  the  tree  stake.  A  second 
and  a  third  stake  are  next  driven  in  the  ground  in  the  apex  of  each 
end  notch,  taking  care  to  set  each  one  firmly  so  that  there  can  be  no 
movement  of  the  board  between  them.  The  board  is  then  removed, 
the  tree  stake  pulled  up,  and  the  hole  dug.  In  planting,  the  board  is 
replaced  between  the  end  stakes  and  the  trunk  of  the  tree  is  held  in 
the  notch  formerly  occupied  by  the  tree  stake.  With  the  roots  in  the 
proper  place,  the  tree  is  planted  and  the  board  removed.    The  tree  is 


Bulletin  328  prune  GROWING  IN  CALIFORNIA  7 

thus  planted  in  the  position  formerly  occupied  by  the  tree  stake,  and 
consequently  in  line  with  every  other  tree. 

Intercrops  may  be  used  in  the  young  orchard.  The  trees  do  not 
need  all  the  space  between  rows  for  several  years.  Intercropping  is 
desirable  from  the  standpoint  of  producing  an  income  before  the  trees 
come  into  bearing.  The  crops  used  for  this  purpose  are  usually  straw- 
berries, peas,  tomatoes,  or  some  other  truck  crop.  The  intercrop  should 
not  be  planted  closer  than  four  feet  from  the  row  the  first  year,  or 
closer  than  six  feet  thereafter.  Grain  or  hay  is  undesirable.  Very 
few  instances  of  growing  young  trees  in  alfalfa  have  ever  been  suc- 
cessful. Intercropping  should  not  be  practiced,  as  a  rule,  more  than 
three  years.  At  the  end  of  this  time  the  trees  need  more  room,  and, 
furthermore,  cast  so  much  shade  that  the  intercrops  do  not  grow  well. 

Effort  should  be  made  to  keep  the  young  trees  growing  vigorously. 
The  plowing  and  subsequent  cultivations  for  the  intercrops  are  usually 
sufficient  for  the  trees.  Where  berries  are  the  intercrop,  the  frequency 
of  irrigation  and  the  tramping  of  the  ground,  when  wet,  by  the  pickers 
may  cause  the  soil  to  puddle  or  run  together.  This  condition  should 
be  avoided  if  possible  by  irrigating  soon  enough  after  one  picking  to 
allow  the  soil  to  dry  out  before  the  next  picking.  Diseases  and  insects 
should  be  held  in  check,  as  described  later  in  this  circular. 

CARE  OF  MATUEE  ORCHARD 

Mature  prune  orchards  are  usually  plowed  in  the  spring  shortly 
before  blossoming.  The  exact  time  depends  upon  the  season,  danger 
of  attack  from  thrips,  amount  of  rainfall,  and  the  equipment  available 
to  the  grower.  After  plowing,  the  orchard  is  ordinarily  worked  with 
the  harrow  and  disc  until  the  surface  soil  is  loose  and  fine.  Culti- 
vation is  practiced  at  intervals  throughout  the  season  whenever  neces- 
sary. The  work  is  done  either  by  horses  or  by  tractors.  In  recent 
years  several  satisfactory  orchard  tractors  have  been  perfected,  and 
these  are  coming  into  extensive  use.  A  thorough  cultivation  is  given 
after  each  irrigation.  Immediately  before  harvest  the  ground  is  either 
planked  or  cultivated  with  a  fine-toothed  implement  to  make  the  sur- 
face as  even  as  possible  to  facilitate  picking,  which  usually  begins  in 
August. 

The  harvest  season  usually  extends  into  October.  As  soon  as  pos- 
sible after  the  last  picking,  the  best  growers  give  the  orchard  an  irri- 
gation. A  cover  crop  is  then  sown  as  soon  as  the  land  can  be  worked. 
Various  cover  crops  are  used.  Some  growers  simply  let  the  native 
vegetation  grow.     Others  use  vetch,  field  peas,  bur  clover,  Melilotus 


8  UNIVERSITY   OF    CALIFORNIA EXPERIMENT    STATION 

indica,  barley,  or  rye.  Melilotus  indica  gives  evidence  of  being  the 
best  crop  to  grow  if  it  can  be  seeded  early.  Otherwise  barley,  rye,  or 
barley  and  vetch  are  sometimes  used.  Vetch  is  often  attacked  and 
stunted  by  aphids. 

The  various  cover  crops  are  seeded  at  approximately  the  following 
rates  per  acre:  vetch,  60  pounds;  bur  clover,  20  pounds;  field  peas, 
80  pounds;  Melilotus,  20-30  pounds;  barley,  50  pounds;  rye,  50 
pounds ;  barley  and  vetch,  20  and  40  pounds,  respectively. 

There  is  a  difference  of  opinion  among  growers  as  to  the  value  of 
commercial  fertilizers  in  prune  orchards.  Furthermore,  there  are  no 
definite,  reliable  experimental  data  on  the  subject.  Barnyard  manure 
is  used  whenever  available.  Its  effect  is  usually  very  marked  on  the 
cover  crop,  and  undoubtedly  it  improves  the  physical  condition  of  the 
soil.  Actual  increases,  however,  in  crop  or  growth  of  the  trees,  par- 
ticularly in  old  orchards,  may  not  be  noticeable  until  after  several 
yearly  applications  have  been  made.  The  only  recommendations  which 
can  be  made  at  this  time  are  that  the  grower  who  believes  his  trees 
need  fertilizer  lay  out  a  small  block  of  trees  and  note  the  effect  of  the 
various  fertilizers  for  a  few  years  in  comparison  with  trees  not  so 
treated. 

Pruning  is  usually  begun  when  the  leaves  fall  and  is  continued 
throughout  the  winter  whenever  the  weather  permits.  Oil  sprays, 
when  used,  are  best  applied  during  January  and  February.  If  lime- 
sulphur  is  used,  it  is  put  on  in  early  spring  before  the  buds  swell. 

TOPWOEKING 

It  is  often  desirable  in  caring  for  an  old  orchard  to  change  the  kind 
and  variety  of  fruit  grown.  This  is  accomplished  by  grafting  and  is 
commonly  spoken  of  as  topworking.  Almonds,  peaches,  and  even 
apricots  may  be  grafted  with  prunes,  or  one  variety  of  prunes  may  be 
changed  to  another.  The  success  of  such  an  operation  will  depend 
upon  the  general  condition  of  the  trees  to  be  grafted  and  the  skill  of 
the  man  doing  the  grafting.  The  first  of  these  is  by  far  the  more  im- 
portant of  the  two.  It  is  usually  easy  enough  to  secure  the  services 
of  a  competent  grafter,  or  the  grower  may  do  the  work  himself.  A 
careful  inventory  of  the  trees  to  be  grafted  should  be  made.  The 
number  of  missing  trees,  as  well  as  the  old  and  the  dying  trees,  should 
be  noted  in  order  to  determine  whether  it  would  be  better  to  graft  or 
to  pull  out  the  old  trees  and  replant.  Ordinarily,  almond  and  apricot 
trees  may  be  grafted  over  profitably  up  to  15  or  20  years  of  age,  pro- 
vided they  are  in  good  condition.  It  is  doubtful  whether  it  would  pay 
to  graft  peaches  that  are  much  over  10  years  of  age. 


Bulletin  328  PRUNE  GROWING  IN  CALIFORNIA  9 

Topworking  may  be  accomplished  by  grafting  or  budding.  The 
former  is  the  more  common  method,  except  where  peaches  are  to  be 
worked  over.  Peaches  are  best  changed  over  by  cutting  back  heavily 
and  budding  into  the  new  wood.  Usually  the  cleft  or  the  bark  graft 
is  used.  With  young  trees  the  whip  graft,  usually  employed  in  nur- 
series, may  be  used.  In  budding,  the  top  of  the  old  tree  is  cut  off.  The 
buds  are  then  inserted  in  the  young  growth  that  grows  out  from  be- 
low the  cut. 


Fig.  1 — The  cleft  is  made  with  a  grafting  chisel. 

The  procedure  in  grafting  over  an  old  tree  is  briefly  as  follows : 
Select  the  branches  to  be  grafted.  These  should  show  straight 
growth  and  should  be  preferably  not  more  than  2%  inches  in  diameter. 
The  number  to  be  grafted  on  each  tree  will  depend  upon  its  size.  With 
a  tree  6  or  7  years  old,  3  or  4  branches  will  be  sufficient.  With  older 
trees,  it  will  probably  be  necessary  to  use  a  larger  number  and  to  graft 
higher  up  than  in  the  case  of  younger  trees. 

The  selected  branches  are  sawed  off.  With  a  grafting  chisel,  a  cleft 
is  made  as  shown  in  Fig.  1.  The  cleft  is  held  open  by  the  wedge  on 
the  chisel  (Fig.  2).  The  scions  are  then  cut  wedge-shape  as  shown  in 
Fig.  3.  The  cut  surface  of  the  scion  should  be  about  l1/^  to  1%  inches 
long  for  the  usual  sized  scion  and  should  be  a  true  flat  surface.    The 


Fig.  2 — Wedge  inserted  to  hold  the  cleft  open  until  the  scions  are  cut  and 
placed  in  position. 


Fig.  3 — Scions  in  place  and  ready  for  waxing.     Scions    (on  right)    cut  and 
ready  for  insertion. 


Bulletin  328 


PRUNE  GROWING  IN  CALIFORNIA 


11 


scions  are  then  inserted,  one  at  either  end  of  the  cleft  (Fig.  3),  with 
the  thin  green  cambium  of  the  scion  in  contact  with  the  cambium  of 
the  stock.  Some  grafters  set  the  scions  slightly  on  a  slant  to  make  cer- 
tain that  the  respective  cambiums  touch.  When  the  wedge  is  removed, 
the  scions  are  tightly  gripped  in  the  stock.  All  cut  surfaces  of  stock 
and  scion  are  then  completely  covered  with  grafting  wax  (Fig.  4). 
Instead  of  splitting  large  branches  (those  3  to  3%  inches  in  diameter), 


Fig.  4 — Different  forms  of  grafts  after  waxing. 

it  is  best  to  notch  them  with  a  coarse  saw,  setting  the  scions  one  inch 
apart  to  facilitate  healing  of  the  wound. 

At  the  end  of  the  first  season,  if  more  than  one  of  the  scions  on  the 
stock  grow,  the  superfluous  ones  are  cut  back  to  stubs  an  inch  long. 
These  greatly  help  the  healing  process  where  the  old  branch  was  cut 
off.  The  growth  made  by  the  scions  is  thinned  out  lightly  the  first  year 
and  thereafter  and  the  cut  surfaces  carefully  rewaxed  or  covered  with 
asphaltum  until  healed. 

Not  all  prunes  make  strong  unions  with  all  stocks.  French  prunes 
have  a  strong  affinity  for  almond  and  peach  but  often  do  not  grow 
well  on  apricot.  The  Sugar  prune,  on  the  other  hand,  is  reported  as 
uniting  with  apricot  as  well  as  with  almond  and  peach.  Robe  de 
Sergeant  does  not  unite  satisfactorily  with  almond  on  nursery  trees. 


12 


UNIVERSITY   OF    CALIFORNIA EXPERIMENT   STATION 


PEUNING 


The  young  prune  tree  as  received  from  the  nursery  is  usually  a 
straight  whip  without  lateral  branches.  After  planting,  this  whip 
should  be  cut  back  to  the  point  at  which  the  head  is  to  be  formed.    The 


Fig.  5 — Branches  should  be  spaced  up  and  down  on  the  trunk  to  avoid  weak 
crotches. 


best  point  at  which  to  make  this  cut  is  from  24  to  30  inches  from  the 
ground.  This  pruning  permits  the  formation  of  a  low-headed  tree 
with  ample  space  between  the  main  scaffold  branches.  The  position 
of  the  head  is  largely  determined  by  this  first  pruning.  If  the  cut  is 
made  too  high,  a  high-headed  tree  will  result;  if  too  low,  the  main 


Bulletin  328 


PRUNE  GROWING  IN   CALIFORNIA 


13 


scaffold  branches  will  be  too  close  together.  A  few  nursery  prune  trees 
have  lateral  branches  developed.  The  head  may  be  formed  from  these 
laterals  at  once  if  they  are  suitably  located  on  the  trunk.  For  spacing 
these  branches,  the  directions  given  in  the  following  paragraph  for 
pruning  one-year-old  trees  should  be  followed. 

During  the  first  season,  time  may  be  saved  in  shaping  the  tree  by 
proper  spring  pinching.  This  pinching  is  solely  for  the  purpose  of 
training  the  young  tree.    It  should  not  be  confused  with  the  summer 


Fig.  6 — The  young  tree  should  have  the  branches  evenly  spaced  around  the 
trunk.  This  photograph,  taken  from  directly  above  the  tree,  shows  an  ideal 
arrangement. 


pruning  sometimes  given  to  mature  trees,  which  has  for  its  purpose  an 
attempted  correction  of  the  bearing  habit.  Summer  pruning  of  the 
young  tree  should  be  restricted  to  pinching  back  undesirable  branches 
early  in  the  season.  It  is  a  distinct  advantage  to  suppress  branches 
at  this  time  which,  if  left,  would  have  to  be  removed  in  the  winter 
season.  Moreover,  growth  is  encouraged  in  the  branches  selected  to 
remain.  The  method  of  pruning  the  young  tree  described  in  the  pre- 
ceding paragraph  is  used  solely  for  the  purpose  of  hastening  the  process 
of  building  up  the  framework  in  the  shortest  possible  time.  Top- 
worked  trees  during  the  first  season  should  be  pinched  back  in  a  manner 
similar  to  that  just  described  for  young  trees. 


14 


UNIVERSITY   OF    CALIFORNIA EXPERIMENT    STATION 


Fi„   7—Two-year-old  French  prune  tree  which  was  cut  back  the  first  winter. 
See  Fig.  8. 


Bulletin  328 


PRUNE  GROWING  IN   CALIFORNIA 


15 


•: 


Cs 


£ 


Fig.  g — The  same  tree  as  in  Fig.  7,  after  pruning  by  thinning  out  branches 
where  too  thick.    This  method  of  pruning  is  best  with  French  prunes. 


16 


UNIVERSITY   OF    CALIFORNIA EXPERIMENT    STATION 


// 


Yig.  9 — Three-year-old  French  prune  tree  which  had  been  thinned  the  pre- 
vious winter.  Note  the  fruit  spurs  already  formed  on  the  main  branches.  See 
Fig.  10. 


Bulletin  328 


PRUNE  GROWING  IN  CALIFORNIA 


17 


Fig.  10 — This  three-year-old  French  prune  tree  (see  Fig.  9)  was  thinned  to 
permit  of  full  development  of  remaining  branches. 


18  UNIVERSITY   OF    CALIFORNIA — EXPERIMENT   STATION 

The  pruning  given  the  first  winter  season  determines  the  form  of 
the  young  tree.  Where  no  summer  pinching  has  been  given,  the  first 
shaping  is  done.  If  the  tree  has  been  summer  pruned,  however,  the 
work  at  this  time  is  materially  lessened.  The  scaffold  branches  chosen 
at  this  time  ordinarily  remain  throughout  the  life  of  the  tree.  The 
utmost  care  should  be  used  to  prevent  mistakes.  The  best  form  is 
built  from  three  scaffold  branches  (Fig.  5).  Some  growers  use  four 
or  even  five  branches,  but  in  most  cases  this  method  brings  the  branches 
too  close  together,  necessitating  the  removal  of  one  or  more  large  limbs 
later.  It  is  desirable  to  have  the  first  branch  12  to  15  inches  from  the 
ground,  with  the  remaining  two  successively  6  to  8  inches  above  the 
one  below.  These  branches  should  be  evenly  spaced  around  the  trunk 
so  that  no  two  come  out  from  the  same  side  of  the  trunk.  In  looking 
down  upon  the  tree  from  directly  above,  the  three  branches  should 
divide  the  space  evenly  (Fig.  6) .  The  three  branches  chosen  to  remain 
should  be  cut  to  approximately  15  to  30  inches  from  their  point  of 
attachment  to  the  main  trunk.  Branches  not  used  for  the  framework 
should  be  cut  off,  except  the  short  growths  on  the  lower  parts  of  the 
tree,  which  should  be  left  to  shade  and  protect  the  trunk  from  the  sun. 
No  further  cutting  back,  as  a  rule,  should  be  done  after  the  first  winter. 

Formerly  the  grower  was  advised  to  cut  the  tree  back  for  two  or 
three  years  in  order  to  make  it  "stocky."  Experiments  at  the  Uni- 
versity Farm  show  that  this  is  not  advisable.  Prune  trees  which  were 
not  cut  back  were  larger  in  diameter  and  had  greater  spread  than  those 
which  had  been  cut  back  annually.  Summer  pruning,  consisting  of 
the  removal  of  water  sprouts  from  the  center  of  the  tree  and  of  suckers 
from  around  the  base,  is  advisable  during  the  second  summer.  "Winter 
pruning,  during  the  second  winter,  should  consist  of  cutting  out 
branches  where  too  close  together  and  the  removal  of  crossing  or  rub- 
bing branches  (Figs.  7  and  8). 

Pruning  during  the  third  and  fourth  seasons  should  be  a  process 
of  thinning  (Figs.  9  and  10).  Consideration  should  be  given  to  the 
location  and  room  allowed  for  development  of  each  branch.  Correction 
of  wayward  limbs  should  be  made  by  cutting  to  a  lateral  growing  in 
the  desired  direction.  Care  should  be  exercised  to  prevent  the  top 
from  becoming  too  dense.  Sunlight  is  essential  for  growth  and  for 
development  of  fruit  spurs  on  the  main  branches.  On  the  other  hand, 
it  is  not  necessary  to  maintain  the  top  of  the  prune  tree  as  open  as  in 
the  case  of  apricots  or  peaches. 

After  the  first  few  years,  the  tendency  among  many  growers  is  to 
practically  cease  pruning  (Fig.  11).  No  ill  effects  are  noticeable  from 
this  practice  for  several  years.     In  the  meantime  the  spurs  in  the 


Bulletin  328  PRUNE  GROWING  IN  CALIFORNIA  19 

interior  portions  of  the  tree  are  killed  by  lack  of  sunlight,  and  the 
bearing  area  is  forced  outward  until  most  of  the  fruit  is  produced  by 
a  comparatively  thin  shell  of  bearing  wood  on  the  outside  of  the  tree. 
A  severe  pruning  must  then  be  given  to  admit  light  to  the  interior 
portions  of  the  tree  in  an  effort  to  reestablish  bearing  wood  in  those 
portions.  This  process  is  commonly  spoken  of  as  ' '  opening  up ' '  a  tree, 
and  is  unnecessary  if  the  tree  is  given  proper  pruning  from  the  start. 

The  pruning  of  mature  prune  trees  is  essentially  a  thinning  process. 
Prune  trees  may  be  kept  in  bearing  and  sufficiently  open  to  admit 
sunlight  by  a  very  small  amount  of  pruning.  This  pruning,  however, 
must  be  consistent  and  done  annually.  At  the  University  Farm  a 
small  block  of  French  prunes  have  been  pruned  in  this  way  for  a 
number  of  years  (Fig.  12).  The  amount  of  wood  removed  each  year 
is  small,  yet  the  spurs  in  the  interior  portions  of  the  trees  are  vigorous 
and  thrifty.  Furthermore,  this  pruning  has  been  done  by  removing 
branches  which  did  not  average  more  than  one-half  inch  in  diameter. 
No  large  cuts  are  necessary. 

A  certain  amount  of  new  growth  each  year  is  desirable.  Without 
it  the  bearing  area  is  constantly  diminished.  Some  spurs  are  lost  each 
year  through  injury  or  natural  causes.  It  is  known  also  that  the 
young  spurs  are  more  productive  than  the  old.  Consequently  new 
growth  upon  which  spurs  are  produced  is  essential  to  continued  maxi- 
mum production.  The  exact  amount  of  new  growth  needed  cannot 
be  stated  definitely.  The  grower  may  be  safe  in  assuming  that  when 
the  new  growth  on  a  majority  of  the  growing  points  falls  below  3  to  6 
inches  for  several  seasons  something  should  be  done  to  bring  about 
greater  growth.  Longer  growth  may  be  obtained  by  a  comparatively 
heavy  thinning  and  by  cutting  back  the  leaders  to  laterals  arising  from 
the  3  and  4  year-old  wood. 

EEJUVENATION  OF  OLD  OECHAEDS 

Through  one  cause  or  another,  old  orchards  sometimes  reach  a 
condition  where  production  is  so  low  that  they  are  no  longer  profitable. 
The  question  which  then  confronts  the  grower  is  what  can  be  done  to 
bring  them  back  into  profitable  production.  If  the  trees  are  appar- 
ently sound,  that  is,  not  affected  with  heart  rot  or  otherwise  growing 
under  adverse  conditions,  such  as  shallow  soil  underlain  with  hardpan, 
lack  of  irrigation  water,  or  severe  infection  of  crown  gall,  they  may 
often  be  rejuvenated  by  proper  cultural  methods.  Sometimes  a  thor- 
ough spraying,  as  discussed  later,  will  destroy  the  parasites  which  are 
stunting  their  growth.    If  the  bearing  area  is  much  reduced,  a  more 


20 


UNIVERSITY   OF    CALIFORNIA — EXPERIMENT   STATION 


or  less  heavy  pruning  may  also  be  necessary.  This  pruning,  in  some 
cases,  should  consist  of  a  removal  of  dead  wood  and  a  heavy  thinning 
of  the  remaining  branches.  Satisfactory  results  in  rejuvenation  of 
old  prune  trees  that  were  not  in  a  dying  condition  have  been  secured 


Fig.  11 — Twelve-year-old  French  prune  tree  which  has  not  been  pruned  for 
six  years.  The  top  is  too  dense  and  the  spurs  in  the  interior  are  losing  their 
vigor. 


Bulletin  328 


PRUNE  GROWING  IN  CALIFORNIA 


21 


by  a  careful  thinning  out  of  entire  branches  and  parts  of  branches  all 
over  the  tree,  especially  at  the  top,  so  as  to  admit  light  to  the  interior. 
Rejuvenation  of  old  orchards  upon  shallow  soils  where  irrigation 
facilities  are  lacking  is  more  difficult,  or  may  even  be  impossible.  If, 
upon  examination,  crown  gall  is  found  to  be  serious,  it  is  unlikely  that 
any  method  of  attempted  rejuvenation  will  be  successful.     Most  old 


Fig.  12 — Twelve-year-old  French  prune  tree  which  has  been  thinned  lightly 
each  year.  The  top  is  open  enough  to  admit  sunlight  to  the  central  portions, 
prolonging  the  life  of  the  spurs  on  the  main  branches. 


orchards  that  have  been  neglected  will  be  found  to  be  infected  with 
the  various  heart  rots,  and  will  also  show  evidence  of  more  or  less  sun- 
burn. Both  troubles  very  greatly  shorten  the  life  of  the  trees.  A 
rather  careful  census  of  the  orchard  should  be  taken  to  determine  the 
extent  of  these  troubles  before  a  rejuvenation  program  is  adopted. 
No  rules  can  be  laid  down  for  the  general  rebuilding  of  old  orchards. 
Each  orchard  must  be  decided  on  its  own  merits,  giving  due  consid- 
eration to  the  factors  outlined  above. 


22  UNIVERSITY   OF    CALIFORNIA— EXPERIMENT    STATION 


IRRIGATION 

Irrigation  practices  with  prunes  vary  greatly  in  the  different  sec- 
tions of  the  state.  Most  growers  who  have  the  facilities  irrigate  once 
or  twice  during  the  growing  season.  Many  have  also  adopted  the  plan 
of  giving  an  additional  irrigation  as  soon  as  possible  after  the  last 
picking.  The  number  of  irrigations  during  the  growing  season  is 
greatly  influenced  by  the  rainfall  of  the  preceding  winter.  If  the  rain- 
fall has  been  light,  two  irrigations  are  usually  given,  and  in  some  cases 
three.  On  the  other  hand,  after  a  season  of  heavy  rainfall  one  irri- 
gation is  often  considered  sufficient.  Many  foothill  orchards  are  grown 
with  no  water  other  than  that  received  from  rainfall  during  the  winter. 
This  practice,  however,  is  usually  the  result  of  necessity  rather  than 
of  choice.  From  observation  of  many  successful  orchards,  it  would 
seem  that  in  the  central  coast  counties  and  interior  valley  sections  a 
program  calling  for  three  irrigations  is  best.  The  first  irrigation 
should  be  given  late  in  May  or  early  in  June ;  the  second  in  July,  just 
before  propping  the  trees;  and  the  third  immediately  after  the  crop 
is  harvested,  preferably  early  in  October  while  the  leaves  are  still  green 
and  active.  In  the  northern  coast  counties,  where  the  rains  continue 
until  late  in  the  season,  the  first  irrigation  may  be  unnecessary.  Like- 
wise, in  sections  having  early  fall  rains,  the  October  irrigation  may 
be  omitted.  In  the  sections  now  producing  the  bulk  of  the  prune  crop 
it  is  usually  not  safe  to  wait  for  the  autumn  rains.  Water  should  be 
applied  while  the  foliage  is  still  on  the  trees. 

Lack  of  sufficient  moisture  in  the  soil  during  the  growing  season  is 
not  shown  in  the  appearance  of  the  prune  tree  until  the  drought  con- 
ditions have  become  very  severe.  Some  other  fruit  trees  indicate  dry 
soil  conditions  readily.  Apricot  trees  show  it  by  a  wilting  of  the  leaves 
during  midday,  and  almond  trees  indicate  the  same  trouble  by  shed- 
ding a  part  of  their  foliage.  The  only  safe  plan  to  follow  is  to  make 
an  examination  of  the  moisture  condition  of  the  soil  itself.  This  may 
be  done  with  the  aid  of  a  soil  auger.  Ordinarily  if -the  top  four  to  six 
feet  of  soil  is  fairly  moist,  the  tree  will  not  suffer. 

An  insufficiency  of  moisture  is  sometimes  shown  in  young  trees  by 
formation  of  terminal  buds  and  stoppage  of  length  growth  before  mid- 
season.  Young  French  prune  trees  at  the  University  Farm  during 
the  season  of  1920  stopped  growth  early  in  June.  Investigation  showed 
the  soil  to  be  very  dry.  New  growth  started  immediately  after  the 
trees  were  irrigated.  Cessation  of  length  growth,  however,  is  not 
always  a  certain  indication  of  lack  of  water  in  the  case  of  mature  trees. 
Bearing  trees,  in  addition  to  adding  new  growth,  are  also  maturing  a 


Bulletin  328  prune  GROWING  IN  CALIFORNIA  23 

crop  of  fruit.  They  ordinarily  cease  length  growth  earlier  than 
younger  trees.  Lack  of  moisture  throughout  the  season  may  also  be 
shown  by  the  small  size  of  the  prunes.  This  last  indication  is  of  service 
only  in  avoiding  the  same  difficulty  the  following  season. 

Most  growers  who  irrigate  agree  as  to  the  value  of  an  early  summer 
irrigation.  Late  summer  irrigation,  just  before  setting  the  props 
under  the  trees,  is  not  so  universally  favored,  because  it  is  believed  to 
cause  cracking  of  the  prunes.  Experiments  being  carried  on  by  the 
College  of  Agriculture  indicate  that  cracking  of  prunes  is  not  due  to 
irrigation.  During  the  season  of  1920  an  exceptionally  heavy  crop  of 
prunes  in  the  Boag  &  Earle  orchard  near  Mountain  View,  Santa 
Clara  County,  was  irrigated  late  in  July.  The  prunes  produced,  in 
the  opinion  of  several  growers,  were  larger  than  would  otherwise  have 
been  the  case,  and  there  was  no  trouble  from  cracking.  These  results 
indicate  that  late  irrigation  does  not  cause  cracking  if  the  prunes  are 
kept  growing  constantly.  The  best  results  are  obtained  if  an  ample 
supply  of  water  can  be  maintained  in  the  soil  throughout  the  season. 
This  condition  can  be  approximated  by  the  two  irrigations  during  the 
summer.  In  a  few  sections  where  orchards  are  situated  on  land  kept 
moist  by  seepage  during  the  growing  season  watering  during  the 
summer  is  probably  unnecessary. 

Tree  props  may  be  rendered  unnecessary  by  installing  a  system  of 
wire  braces  in  the  trees.  A  No.  14  galvanized  wire  should  run  from 
each  branch  to  a  stout  ring  in  the  center  of  the  tree.  The  wires  are 
made  fast  to  the  branches  with  small  fence  staples,  which  are  drawn 
snugly  down  against  the  bark.  These  are  quickly  grown  over  and  do 
not  pull  out. 

The  October  irrigation  before  the  leaves  fall  is  of  great  value  for 
the  succeeding  season's  crop.  Part  of  the  large  crop  of  1919  was  un- 
doubtedly due  to  the  heavy  rains  of  the  previous  September.  Bud 
studies  by  the  Pomology  Division  have  shown  that  under  favorable 
conditions,  such  as  are  brought  about  by  early  fall  irrigation,  differ- 
entiation of  leaf  buds  to  fruit  buds  continues  throughout  October  and 
November  and  even  into  December.  The  value  of  water  applied  in 
October  is  shown  by  the  increased  bloom  the  following  spring. 

Irrigation  water  is  usually  applied  to  prune  orchards  by  the  basin 
or  check  method.  Under  certain  conditions  the  furrow  system  may  be 
preferable.  An  average  irrigation  is  probably  approximately  3  or  4 
acre  inches.  A  common  mistake  in  irrigating  is  to  regulate  the  amount 
of  water  put  on  the  land  by  an  approximate  estimate,  rather  than  by 
actually  measuring  the  depth  to  which  the  water  penetrates.  Most 
irrigators  would  be  surprised  if  they  knew  the  comparatively  shallow 


24  UNIVERSITY   OF   CALIFORNIA — EXPERIMENT   STATION 

penetration  of  average  irrigations.  To  be  effective  over  a  period  of 
time  an  irrigation  should  wet  the  soil  to  a  depth  of  at  least  5  or  6  feet. 
From  lighter  irrigations  too  large  a  proportion  of  water  applied  is  lost 
by  evaporation  from  the  top  layers  of  soil  and  not  enough  reaches  the 
roots. 

POLLINATION 

The  status  of  prunes  as  regards  their  ability  to  set  fruit  with  their 
own  pollen  is  fairly  well  known.  The  French  and  the  Sugar  prunes 
are  self-fertile  and  bear  abundant  crops  when  growing  in  large  blocks 
of  one  variety.  The  Imperial  and  Robe  de  Sergeant  are  self-sterile 
and  should  be  interplanted  with  another  variety.  Any  combination 
of  the  four  principal  prune  varieties  is  satisfactory  for  pollination 
purposes.  Experiments  have  demonstrated  the  value  of  bees  in  the 
prune  orchards  of  the  state.2  These  experiments  are  amply  substanti- 
ated by  local  observations  in  practically  every  orchard  section. 

DISEASES 

Brown  Rot. — Brown  rot,  until  recently,  has  not  been  generally 
found  on  prune  trees,  except  in  some  of  the  mountain  or  coastal  sec- 
tions. During  the  season  of  1920  a  considerable  amount  of  this  disease 
was  noticed,  both  on  the  blossoms  early  in  the  season  and  later  on  the 
fruit  itself.  This  fungus  has  long  been  known  as  a  disease  of  stone 
fruits,  and  at  various  times  has  been  serious  in  parts  of  California. 
Outbreaks  have  occurred  from  time  to  time,  principally  on  cherries  and 
apricots. 

During  the  spring  of  1920  the  Deciduous  Fruit  Station  at  Mountain 
View,  Santa  Clara  County,  successfully  controlled  brown  rot  on  apri- 
cots. The  same  disease  attacks  the  prune,  hence  similar  measures  of 
control  should  be  used.  Lime-sulphur  (1  gallon  to  9  gallons  of  water) 
or  Bordeaux  (4  pounds  copper  sulphate,  5  pounds  stone  lime,  50  gallons 
water)  were  equally  successful  when  used  against  this  disease.  The 
spray  should  be  applied  in  early  spring  when  the  first  few  buds  are 
showing  white.  No  injury  will  result  if  the  first  blossoms  are  actually 
open. 

If  the  rot  on  the  fruit  itself  continues  as  serious  as  during  the 
season  of  1920,  it  may  become  necessary  to  spray  for  this  form  also. 
Experiments  are  now  in  progress  for  controlling  both  the  spring  and 
summer  forms  on  prunes,  but  no  recommendations  can  be  made  at  this 
time. 


2  See  Calif.  Agr.  Exper.  Sta.  Bulls.  291  and  310. 


Bulletin  328  PRUNE  GROWING  IN  CALIFORNIA  25 

Crown  Gall. — Crown  gall  is  a  bacterial  disease  working  on  the 
crown  and  roots  of  most  tree  fruits.  It  is  severe  on  all  stone  fruits, 
although  almond  and  peach  stocks  are  usually  considered  somewhat 
more  susceptible  than  myrobalan.  Being  a  root  disease,  it  is  difficult 
to  eradicate.  The  organism  which  causes  it  is  probably  present  in  the 
soil  of  all  our  fruit  sections.  Nothing  can  be  done  to  keep  it  from 
attacking  trees  already  planted.  Fortunately,  after  a  tree  is  well 
established  a  certain  amount  of  crown  gall  appears  to  do  but  little 
damage.  In  fact,  in  some  cases  fruitfulness  is  indirectly  caused  by  it. 
Severe  infections  are  shown  by  stunted  growth  and  unthrifty  appear- 
ance of  the  tree.  If  the  galls  are  large  and  near  the  surface  on  the 
large  roots,  they  may  be  chiseled  off  and  the  wounds  painted  with 
Bordeaux  paste.  It  is  somewhat  doubtful  if  this  practice  is  worth  its 
cost.  Young  trees,  however,  should  be  carefully  examined  for  galls, 
and  if  any  are  found  the  tree  should  be  rejected  for  planting.  In  this 
way  the  young  tree,  even  if  planted  in  soil  where  the  organism  exists, 
is  enabled  to  secure  a  good  start  before  being  handicapped  by  the 
disease. 

Heart  Rots. — Decay  of  the  heart  wood  of  prune  trees  may  be  caused 
by  several  different  fungi.  These  organisms  must  first  gain  entrance 
to  the  heart  wood  through  some  exposed  wound.  Old  pruning  wounds 
are  the  chief  starting  points.  After  the  fungus  has  once  gained  en- 
trance, it  is  practically  impossible  to  stop  its  spread.  Sooner  or  later 
the  large  branches  break  down  under  heavy  loads  or  strong  winds. 
Prevention  is  the  only  method  of  control.  All  wounds  over  one  inch 
in  diameter  should  be  covered  with  asphaltum,  roofing  paint,  or  some 
other  material  that  adheres  well. 

Oak  Fungus. — Oak  fungus  is  one  of  the  most  serious  diseases  the 
prune  grower  has  to  face.  It  is  known  by  other  names,  such  as  root  rot, 
toadstool  disease,  and  Armdllaria.  The  causative  organism  is  a  fungus 
known  as  Armillaria  mellea,  which  works  underground  on  the  roots, 
and  which  is  supposed  to  have  its  origin  on  the  roots  of  infected  oak 
trees  that  formerly  occupied  the  ground. 

The  first  indication  of  its  presence  is  usually  the  death  of  a  tree, 
often  in  midsummer,  apparently  from  no  cause.  In  succeeding  seasons 
more  trees  die.  The  infected  area  is  usually  circular  in  form.  The 
disease  works  rather  slowly,  but  in  time  completely.  In  a  few  years 
it  may  destroy  trees  over  a  quarter  of  an  acre  of  land. 

Roots  of  trees  which  have  been  killed  by  this  fungus  show  a  white 
fan-shaped  mycelium  under  the  bark  and  are  characterized  by  a  pro- 
nounced musty  odor.  On  the  surface  of  the  roots  slender  black  or  pur- 
ple strands  may  often  be  seen.    These  are  the  hyphal  threads  by  which 


26  UNIVERSITY   OF    CALIFORNIA EXPERIMENT    STATION 

the  fungus  spreads.  These  hyphal  threads  or  mycorrhiza  are  able  to 
push  through  the  soil  for  a  considerable  distance,  possibly  several  feet. 
When  they  come  in  contact  with  the  roots  of  another  tree  that  tree 
becomes  infected.  This  manner  of  infection  from  root  to  root  and  tree 
to  tree  accounts  for  the  circular  spread  of  the  disease.  During  the 
winter  or  early  spring  after  the  heavy  rains  the  presence  of  the  fungus 
is  often  shown  by  a  clump  of  toadstools  which  are  pushed  up  near  the 
crown  of  the  infected  trees. 

Isolation  of  the  infected  area  is  the  only  method  of  fighting  this 
disease  that  has  as  yet  been  devised.  Roots  from  infected  trees  or  those 
thought  to  be  infected  must  be  kept  from  coming  into  contact  with  those 
of  healthy  trees.  A  trench  dug  around  the  area  to  be  isolated  is  the 
best  method.  This  trench  need  not  be  very  wide  and  just  deep  enough 
to  be  certain  it  is  below  the  root  area.  The  trench  should  be  dug  so 
there  is  at  least  one  row  of  healthy  trees  between  it  and. the  diseased 
area.  This  should  preclude  any  danger  of  finding  diseased  roots  al- 
ready crossing  into  the  area  to  be  guarded.  In  most  cases  a  depth  of 
3  feet  will  be  sufficient  if  the  trench  is  dug  at  least  12  feet  from  a  row 
of  trees,  although  in  some  soils  it  may  have  to  be  4  or  5  feet  deep.  Roots 
entering  the  trench  should  be  cut  off  and  burned. 

This  trench  may  be  left  open,  or  it  may  be  refilled.  If  refilled,  it 
should  be  reopened  every  alternate  season  and  the  roots  which  have 
entered  it  again  cut  off.  Some  growers  prefer,  after  the  trench  is 
opened,  to  build  a  concrete  wall  around  the  diseased  area.  If  no  dis- 
eased roots  have  crossed  the  line  thus  established,  the  concrete  barrier 
should  effectively  stop  further  spread.  The  chief  objection  to  the 
concrete  wall  is  its  cost.  On  the  other  hand,  a  ditch  left  open  occasions 
great  inconvenience  in  irrigation  and  cultivation. 

From  time  to  time  experiments  have  been  tried  in  disinfecting  the 
soil  with  various  substances  having  fungicidal  properties,  but  none  of 
these  have  been  markedly  successful.  The  ultimate  solution  of  the 
problem  seems  to  be  a  root  stock  resistant  to  this  fungus,  and  the 
Deciduous  Fruit  Station  of  the  University  of  California  is  giving  a 
great  deal  of  attention  to  a  study  of  this  possibility.  From  the  very 
nature  of  the  disease  it  is  evident  that  positive  results  cannot  be  ex- 
pected for  many  years. 

The  grower  is  then  confronted  with  the  problem  of  what  to  do  with 
the  oak  fungus  areas.  All  the  common  root  stocks  of  the  prune  are 
affected  by  this  disease.  Myrobalan  is  perhaps  somewhat  more  resist- 
ant than  peach  or  almond,  but  conclusive  data  on  this  point  are  lacking. 
Replants  of  prunes  on  any  root  stock  adapted  to  them  are  practically 
certain  of  infection  within  a  very  few  years  after  planting.    There  are 


Bulletin  328  prune  GROWING  IN  CALIFORNIA  27 

three  fruits,  however,  which  are  considered  resistant.  The  experience 
of  many  growers  indicates  that  the  California  black  walnut,  the  pear 
on  French  root,  and  the  fig  are  resistant.  Any  one  of  these  three 
which  is  suited  to  the  district  may  be  planted  without  danger. 

Rust. — Rust  is  of  but  minor  importance  on  prunes.  It  appears  as 
a  reddish  brown  mass  on  the  under  side  of  the  leaves.  Usually  it  is 
not  noticeable  until  late  in  the  season,  after  the  crop  is  picked.  Ordi- 
narily no  control  measures  are  necessary,  as  the  leaves  begin  to  fall 
normally  a  few  weeks  later.  Should  the  disease  be  severe  earlier  in 
the  season,  when  defoliation  would  be  serious,  the  tree  should  be 
sprayed  with  Bordeaux  mixture  (4  pounds  copper  sulphate,  5  pounds 
stone  lime,  50  gallons  water) . 

INSECTS 

Insects  injurious  to  prune  trees,  with  one  exception,  have  not  been 
hard  to  control.  The  exception  is  the  thrips,  but  with  sprays  and  more 
recently  with  a  new  powder  known  as  "Nicodust,"  even  this  trouble- 
some pest  can  be  controlled. 

In  general,  the  insect  pests  of  prune  trees  can  be  divided  into  two 
classes,  the  chewing  insects  and  the  sucking  insects.  The  former  are 
controlled  by  some  form  of  poison  spray,  usually  arsenate  of  lead. 
For  the  latter  a  contact  spray  such  as  lime-sulphur  or  oil  emulsion  must 
be  used.  Borers,  however,  cannot  be  controlled  by  either  of  the  above, 
and  must  be  dug  out  with  a  knife  or  stiff  wire. 

Three  factors  essential  for  a  successful  fight  against  these  pests  are : 
suitable  material ;  thoroughness ;  and  application  at  the  right  time. 
The  first  two  factors  are  well  understood.  The  third  is  often  disre- 
garded, with  the  result  that  the  expense  is  wasted  and  the  grower  loses 
confidence  in  the  operation.  Ordinarily,  for  each  insect  there  is  a 
time,  as  described  later,  when  it  can  be  effectually  controlled.  Spray- 
ing earlier  or  later  than  this  is  often  of  little  or  no  value. 

In  this  circular  only  the  insects  of  common  occurrence  on  the 
prune  are  described. 

Mealy  Plum-Louse  or  Aphid. — These  insects  are  very  small,  light 
green  in  color,  and  usually  covered  with  a  whitish  mealy  substance. 
They  hatch  in  early  spring  from  small  black  eggs  laid  the  previous 
fall  on  the  limbs  of  the  tree  and  immediately  begin  working  on  the 
under  side  of  the  leaf.  If  unchecked,  they  soon  cause  the  leaves  to 
curl,  making  spraying  very  difficult.  A  honeydew  is  exuded  which 
causes  a  blackening  of  the  leaves  and  branches  and,  in  severe  cases,  a 
cracking  of  the  fruit  and  stunting  of  the  trees.    For  control  the  fol- 


28  UNIVERSITY   OF    CALIFORNIA EXPERIMENT    STATION 

lowing  spray3  should  be  used  when  the  insects  first  appear:  soap,  4 
pounds;  Black  Leaf  40,  1  pint;  water,  100  gallons.  This  must  be 
applied  thoroughly,  wetting  the  under  side  of  the  leaves.  A  second 
application  may  be  necessary  in  badly  infested  parts  of  the  orchard. 
Dusting  with  2  per  cent  Nicodust  has  given  promising  results,  and  is 
recommended  for  trial. 

Peach  Tree  Borer. — The  young  borers,  from  eggs  laid  during  the 
summer  months,  principally  from  June  to  September,  enter  through 
the  bark  of  the  tree.  The  work  of  the  borer  can  be  detected  by  the 
gum  and  frass  (a  sawdust-like  material  which  passes  through  the  body 
of  the  insect)  that  is  usually  found  on  the  trunk  between  the  main 
roots  and  the  surface  of  the  ground.  As  yet  no  entirely  satisfactory 
method  of  controlling  this  pest  in  California  has  been  devised  other 
than  the  old  laborious  digging  out  with  a  knife  or  wire.  If  the  orchard 
is  badly  infested,  the  borers  should  be  dug  out  in  the  fall  and  the  trees 
inspected  in  the  spring  for  any  missed  in  the  previous  operation.  Care 
should  be  exercised  in  digging  out  these  pests.  In  cutting  to  find  the 
tunnels,  the  careless  workman,  if  not  cautioned,  may  seriously  injure, 
if  not  completely  girdle,  the  tree.  The  hole  in  the  soil  made  in  this 
work  is  sometimes  left  until  spring  to  save  the  expense  of  reopening. 
This  practice  is  a  questionable  one,  especially  in  sections  where  the 
roots  are  thus  exposed  to  freezing  temperatures. 

A  coat  of  whitewash  applied  to  the  crown  of  the  tree  seems  to  act 
as  a  deterrent  to  the  moth,  and  is  of  some  value.  Asphaltum,  grade  C 
or  D,  applied  hot  to  the  crown  and  upper  portions  of  the  roots,  has 
materially  lessened  the  injury  from  this  insect  in  some  sections. 

Flat-Headed  Apple  Tree  Borer. — Young  prune  trees  are  also  at- 
tacked by  this  insect.  The  larvae  or  worms  feed  in  the  sapwood  at  or 
near  the  crown.  Older  trees  are  often  infested  where  sunburned  or 
injured  by  the  plow  or  cultivator.  The  oval  shape  of  the  burrows  is 
due  to  the  flattened  head  of  the  borer.  The  trees,  particularly  when 
young,  should  be  watched  carefully  and  the  borers  dug  out.  The  bur- 
rows are  indicated  by  discoloration  of  the  bark.  Whitewash  is  a  good 
repellent. 

Canker  worms. — These  insects  are  also  known  as  measuring  worms. 
They  are  quite  variable  in  color  and  usually  appear  early  in  the  grow- 
ing season.  When  present,  they  may  often  be  seen,  particularly  after 
a  heavy  wind,  hanging  suspended  from  the  tree  by  a  silken  cord.  As 
they  eat  the  foliage  of  the  tree,  they  can  be  controlled  by  a  poison 
spray.    Neutral  lead  arsenate  at  the  rate  of  8  pounds  of  paste  or  4 

3  Proved  to  be  successful  by  D.  L.  Currier,  Assistant  Horticultural  Commis- 
sioner of  Santa  Clara  County. 


Bulletin  328  PRUNE  GROWING  IN  CALIFORNIA  29 

pounds  of  powder  to  200  gallons  of  water  should  be  applied  as  soon  as 
the  worms  appear.  It  is  advisable  to  add  4  pounds  of  slaked  slime  to 
the  spray  before  applying.  Only  basic  or  neutral  arsenate  of  lead 
should  be  used.  If  the  orchard  has  been  troubled  with  this  pest  for 
several  years,  banding  the  trees  is  often  advisable.  Tanglefoot,  if  kept 
fresh,  will  prevent  the  female  from  crawling  up  the  trees  to  lay  her 
eggs. 

Red  Humped  Caterpillar. — This  insect  is  named  from  the  red  hump 
on  the  fourth  segment  or  division  of  its  body.  When  full  grown  it 
may  be  from  1%  to  2  inches  long.  It  is  a  serious  pest  in  some  sections, 
and  is  often  found  in  the  central  part  of  the  state.  It  is  found  chiefly 
in  colonies  and  may  defoliate  entire  branches  in  a  very  short  time. 
On  young  trees  the  colonies  may  be  picked  off  by  hand  or  the  infested 
branches  clipped  off  with  pruning  shears.  For  larger  trees,  spraying 
with  arsenate  of  lead,  as  recommended  for  cankerworm,  is  advised. 
Fall  cultivation  close  to  the  trees  aids  in  killing  the  dormant  insect. 

Leaf  Roller. — This  insect,  like  the  two  described  in  the  preceding 
paragraphs,  also  feeds  on  the  foliage.  It  is  readily  distinguished  by 
its  habit  of  rolling  itself  up  in  a  leaf  while  feeding.  A  spray  of  arsen- 
ate of  lead,  as  recommended  for  cankerworms,  should  be  applied  when 
the  worms  first  appear  and  the  foliage  is  still  small.  A  thorough 
spraying  with  miscible  oil  in  late  winter  or  early  spring  will  kill  the 
eggs. 

Red  Spider  or  Mite, — Red  spider  injury  is  generally  shown  by  a 
yellowing  and  premature  dropping  of  the  leaves.  Severe  attacks  cause 
still  further  injury,  such  as  small  size  of  fruit,  weakness  of  fruit  buds, 
and  death  of  the  terminal  growth. 

There  are  two  species  of  red  spider  commonly  found  on  prune  trees, 
the  so-called  almond  mite  and  the  summer  or  two-spotted  mite.  The 
almond  mite  is  about  the  size  of  a  pinhead  and  red  or  reddish  brown 
in  color.  The  eggs  are  minute  and  red  in  color  and  are  laid  on  the 
twigs  and  branches  of  the  tree.  It  is  somewhat  easier  to  control  than 
the  two-spotted  or  summer  mite.  A  crude  oil  emulsion,  as  recom- 
mended below  for  Italian  pear  scale,  effectively  controls  this  pest  in 
most  districts.  Lime-sulphur,  1  gallon  to  9  gallons  of  water,  is  also 
widely  used,  although  it  does  not  appear  to  be  so  effective  as  the  oil 
spray.  The  best  time  for  application  of  the  spray  is  late  winter  or 
early  spring. 

The  two-spotted  or  summer  mite  winters  in  the  ground  or  under 
the  bark.  Its  control,  therefore,  is  almost  entirely  a  summer  measure. 
These  mites  appear  during  the  first  few  weeks  of  warm  weather.    They 


30  UNIVERSITY   OF    CALIFORNIA EXPERIMENT    STATION 

work  on  the  under  side  of  the  leaves,  on  which  they  spin  a  delicate  web 
which  protects  them  and  their  eggs.    The  rate  of  increase  is  very  rapid. 

Control  measures  must  start  as  soon  as  the  mites  appear.  Satis- 
factory results  have  been  obtained  by  using  a  very  fine  dry  sulphur 
at  the  rate  of  from  one-sixth  to  one-third  of  a  pound  to  a  tree.  Some 
growers  who  use  a  power  duster  prefer  to  dilute  the  sulphur  with 
about  50  per. cent  of  air-slaked  or  hydrated  lime  to  prevent  injury  to 
the  foliage.  In  case  of  severe  infestation,  dusting  should  be  continued 
at  intervals  of  from  ten  days  to  two  weeks  until  it  is  apparent  that  the 
insect  has  been  controlled.  In  young  orchards  or  in  windy  locations 
the  use  of  dry  sulphur  has  not  always  been  successful,  because  the 
fumes  liberated  by  the  sulphur  do  not  remain  concentrated  enough  to 
kill  the  insects. 

Where  dusting  cannot  be  employed,  lime-sulphur,  sulphur  paste, 
or  some  form  of  ' '  wettable ' '  sulphur  will  have  to  be  used  as  a  spray. 
Lime-sulphur  solution  ordinarily  should  not  be  used  stronger  than 
1  gallon  to  75  gallons  of  water.  Even  this  dilution  sometimes  causes 
injury  to  the  foliage,  particularly  if  the  spraying  happens  to  be  fol- 
lowed by  a  period  of  high  temperature. 
-  Scale  Insects. — Two  scale  insects  are  of  special  concern  to  the  prune 
grower,  the  brown  apricot  scale  and  the  Italian  pear  scale.  The  former 
is  a  rather  large  hemispherical  scale  easily  seen  on  the  twigs  and  smaller 
branches.  The  latter  is  not  so  conspicuous,  but  is  much  more  harmful. 
It  is  about  the  size  of  a  pinhead,  and  is  usually  found  on  the  larger 
limbs  and  branches,  where  it  is  often  afforded  protection  by  lichens 
and  moss.  By  scraping  away  the  moss,  the  small,  white,  flat  covering 
of  the  scale  can  be  easily  seen.  Under  this  are  the  red  or  dark  red 
bodies  of  the  insects  themselves.  The  presence  of  this  scale  is  generally 
shown  by  the  flattened  appearance  of  the  branches  and  limbs.  This 
condition  will  be  found  principally  on  the  north  side  of  the  limbs, 
where  the  moss  is  thickest.  This  scale  causes  the  death  of  many  limbs 
in  badly  infested  orchards. 

The  brown  apricot  scale  is  easily  controlled  with  a  5  per  cent  dis- 
tillate emulsion  a  miscible  oil,  or  a  12  per  cent  crude  oil  emulsion. 
On  young  trees  the  first  two  sprays  are  usually  preferred,  as  there 
seems  to  be  some  tendency  to  sunburn  when  the  heavier  oil  is  used. 

The  Italian  pear  scale  is  best  controlled  by  the  use  of  the  crude  oil 
emulsion.  A  12  per  cent  emulsion  is  found  satisfactory  in  most  local- 
ities, but  a  few  are  still  using  a  15  per  cent  emulsion.  Where  moss 
and  lichens  are  troublesome,  the  addition  of  from  6  to  8  pounds  of 
caustic  soda  will  easily  remove  them.  Lime-sulphur  at  the  rate  of  1 
gallon  to  9  gallons  of  water  is  still  used  in  some  orchards,  but  it  is  not 


Bulletin  328  PRUNE  GROWING  IN  CALIFORNIA  31 

quite  so  effective  as  the  oils.  Where  lime-sulphur  is  constantly  used, 
however,  the  Italian  pear  scale  seldom  becomes  serious. 

December  to  February  is  the  best  time  for  spraying  for  scale. 
Spraying  too  early  in  the  winter,  when  the  trees  are  dry,  is  often  the 
cause  of  oil  injury  to  the  buds.  Trees  must  not,  however,  be  sprayed 
when  wet,  as  immediately  following  a  rain  for  example,  for  the  spray 
will  run  off  and  not  kill  the  scale.  Ideal  conditions  are  when  the  trees 
are  slightly  damp.  If  spraying  is  thoroughly  done,  it  is  not  necessary, 
under  ordinary  conditions,  to  spray  oftener  than  every  second  or  third 
year. 

Thrips. — The  thrips  or  pear  thrips,  as  it  is  often  called,  is  in  many 
ways  the  most  serious  insect  pest  with  which  the  prune  grower  has  to 
contend.  In  its  different  stages,  this  insect  is  often  responsible  for  the 
scabbing  of  the  fruit  or  the  loss  of  the  crop. 

The  adult  thrips  is  a  winged  insect,  very  small,  slender,  and  almost 
black.  It  appears  late  in  February  or  early  in  March  when  the  prune 
buds  are  beginning  to  swell  and  open.  If  the  thrips  are  numerous, 
many  buds  may  fail  to  open,  or,  if  they  do  open,  the  flowers  appear 
scorched.  They  do  further  injury  by  inserting  their  eggs  in  the  stem 
of  the  young  flower.  This  injury  may  cause  the  flower  to  fall  or  the 
fruit  to  drop  before  maturity.  The  young  thrips  or  white  thrips  hatch 
and  are  most  abundant  during  the  early  part  of  April.  This  form  of 
the  insect,  in  feeding  on  the  young  fruit,  causes  what  is  known  as 
"scabby"  prunes.  The  thrips  later  drop  to  the  ground,  where  they 
work  into  the  hard  soil  below  the  surface  mulch  and  remain  there  until 
the  following  spring.  Pupation  begins  about  September  and  continues 
until  all  have  changed  to  the  adult  or  black  form. 

Heretofore  the  thrips  have  been  combated  with  a  liquid  spray, 
usually  a  li/2  per  cent  oil  emulsion,  to  which  is  added  1  pint  of  Black 
Leaf  40  to  every  200  gallons  of  spray.  Results  with  this  spray  have 
been  fairly  satisfactory.  Recently,  however,  a  new  material,  "Nico- 
dust,"  has  been  used  which  gives  promise  of  being  the  most  effective 
yet  devised.  The  5  per  cent  strength  is  recommended  for  thrips.  It 
should  be  dusted  on  to  the  trees  with  either  a  hand  or  a  power  duster 
when  the  first  black  thrips  appear  in  the  spring.  Ordinarily  from 
one-third  to  one-half  a  pound  of  Nicodust  will  be  sufficient  for  an 
average  prune  tree.  More  than  one  dusting  may  be  necessary,  de- 
pending on  the  number  of  thrips  present.  The  best  time  for  appli- 
cation is  between  9  a.m.  and  6  p.m.  The  Nicodust  volatilizes  best  when 
the  temperature  is  fairly  high.  A  power  duster  is  capable  of  dusting 
20  to  40  acres  a  day,  while  from  3  to  6  acres  may  be  covered  with  the 
ordinary  hand  duster. 


32  UNIVERSITY   OF    CALIFORNIA EXPERIMENT    STATION 

Other  methods  have  been  found  helpful  in  thrips  control.  An 
irrigation  immediately  after  the  crop  is  harvested  kills  many  of  the 
hibernating  thrips.  In  the  spring  some  growers  leave  the  cover  crop 
until  after  the  trees  have  blossomed.  This  practice  seems  to  reduce 
thrips  injury.  It  should  be  remembered,  however,  in  connection  with 
this  latter  practice,  that  a  late-growing  cover  crop  materially  reduces 
the  amount  of  moisture  in  the  soil  available  for  the  trees,  and  it  should 
therefore  not  be  followed  except  where  irrigation  is  possible. 

VARIETIES  OF  PEUNES 

French  {Prune  d'Agen,  Petite  Prune). — The  French  prune  is  the 
most  important  commercial  prune  in  California.  In  speaking  of 
prunes,  the  French  prune  is  meant  unless  other  varieties  are  specifi- 
cally mentioned.  For  all  around  adaptability,  regularity  of  produc- 
tiveness, and  vigor  and  strength  of  trees,  it  is  unsurpassed  in  Califor- 
nia. Its  chief  defect  is  lack  of  size.  During  some  seasons,  and  par- 
ticularly under  stress  of  heavy  loads  in  unirrigated  orchards,  the 
smaller  sizes  predominate.  The  marketing  possibilities  of  the  larger 
prunes  have  been  strikingly  shown  during  the  past  few  seasons.  In 
addition  to  the  excellent  qualities  of  the  tree,  the  French  prune  is  easily 
handled  in  the  dry  yard.  Several  so-called  improved  "strains"  of 
the  French  prune  have  been  introduced  in  the  past  few  years.  Some 
of  these  have  much  promise. 

Robe  de  Sergeant. — This  variety  is  quite  extensively  planted  in 
some  of  the  interior  valley  sections.  When  dried  it  resembles  the 
French,  and  is  usually  sold  as  such.  The  tree  and  fruit,  however,  are 
distinctly  different  from  the  French.  The  leaves  are  a  dark  glossy 
green,  as  distinguished  from  the  yellowish  green  foliage  of  the  French. 
The  fruit  is  not  distinctly  necked,  as  is  the  latter,  and  is  darker  in  color 
when  fully  ripe.  This  variety,  furthermore,  is  self -sterile  and  should 
be  planted  with  the  French  or  Imperial  to  secure  fertilization  of  its 
flowers. 

Imperial. — The  Imperial  is  grown  to  some  extent.  The  fruit  is 
large  and  of  excellent  quality,  but  the  tree  is  a  notoriously  shy  bearer, 
except  in  a  few  favored  localities.  It  seems  to  produce  well  in  the 
counties  north  of  San  Francisco  Bay.  The  tree  when  mature  is  a 
rather  weak  grower.  Spurs  are  usually  borne  in  abundance  and  the 
blossoming  is  profuse.  The  set,  however,  is  usually  light.  The 
branches  are  rather  stiff  and  rigid,  but  are  brittle  and  break  easily. 
The  slow  growth  in  old  trees  may  be  corrected  by  somewhat  heavier 
pruning  than  is  usually  given  the  French  variety.     This  variety  is 


Bulletin  328  prune  GROWING  IN  CALIFORNIA  33 

also  self-sterile  and  should  be  interplanted  with  either  the  French  or 
the  Sugar  variety.  Because  of  its  size,  it  does  not  dry  so  readily  as 
the  French  and  requires  special  attention  in  the  dry  yard.  When 
dried,  the  fruit  is  usually  of  such  large  size  that  it  commands  a  higher 
price  than  most  prunes. 

Sugar. — The  Sugar  prune  is  grown  in  certain  sections,  especially 
the  warmer  interior  valleys.  The  tree  grows  vigorously  when  young, 
but  requires  liberal  pruning  to  maintain  adequate  wood  growth  and 
prevent  overbearing  as  it  grows  older.  The  wood  is  brittle  and  breaks 
easily  under  the  weight  of  ordinary  crops,  often  in  spite  of  propping. 
Heavy  bearing  may  be  said  to  be  one  of  its  greatest  faults.  In  some 
sections  it  is  necessary  to  thin  the  crop  in  order  to  save  the  trees.  How- 
ever, it  has,  in  some  sections,  a  pronounced  tendency  to  alternate  bear- 
ing. The  dried  fruit  is  often  large  but  of  mediocre  quality,  being 
somewhat  coarse  and  stringy.  As  it  ripens  a  week  or  ten  days  earlier 
than  the  French,  it  is  favored  in  some  sections  where  trouble  is  ex- 
perienced in  drying  the  late  pickings  of  the  latter  variety.  The  Sugar 
prune  is  self-fertile  and  may  be  planted  without  regard  to  cross- 
pollination  by  other  varieties. 

CURING 

Most  prunes  grown  in  California  are  sun-dried.  In  recent  years, 
however,  an  increasing  proportion  are  dried  in  evaporators.  Evapor- 
ation, while  more  costly,  assures  the  grower  that  he  can  dry  all  his 
crop  without  regard  to  weather  conditions.  The  recent  increase  in 
popularity  of  evaporation  in  this  state  undoubtedly  was  greatly  stim- 
ulated by  the  disastrous  season  of  1918,  when  a  large  proportion  of 
the  crop  was  spoiled  by  mold  and  fermentaion  following  the  heavy 
rains  early  in  September. 

Whether  sun-dried  or  evaporated,  the  handling  of  the  crop  is  the 
same  up  to  the  point  of  placing  the  prunes  on  the  trays.  The  prunes 
are  generally  allowed  to  fall  to  the  ground  and  are  then  picked  up  and 
put  into  lug  boxes.  In  some  of  the  interior  valley  sections  it  is  neces- 
sary to  shake  the  fruit  from  the  trees  when  it  is  ripe.  Four  or  five 
pickings  during  the  season  are  usually  sufficient.  The  pickers  are  not 
allowed  to  shake  the  trees  until  the  last  picking. 

After  picking,  the  prunes  are  first  taken  to  the  dipper,  where  they 
are  immersed  in  a  hot  solution  of  lye.  The  strength  is  usually  about 
1  pound  of  lye  to  20  gallons  of  water.  The  time  of  immersion  varies 
from  a  few  seconds  to  nearly  a  minute,  depending  upon  the  strength 
and  temperature  of  the  "dip."    Most  growers  aim  to  keep  the  solution 


34  UNIVERSITY   OF    CALIFORNIA EXPERIMENT    STATION 

at  or  near  the  boiling  point  for  French  prunes  and  slightly  lower  for 
Imperials.  The  dip  should  remove  all  the  bloom  or  wax  from  the 
surface  of  the  fruit  and  slightly  check  or  crack  the  skin.  Severe 
checking  is  undesirable  and  causes  a  shriveling  of  the  fruit.  Obser- 
vation will  quickly  indicate  whether  the  prunes  have  been  checked 
sufficiently.  If  the  solution  is  too  strong  or  the  immersion  too  long, 
the  skin  may  be  entirely  removed.  If  it  is  too  weak  or  cold,  the  skin 
of  many  prunes  may  not  be  checked  at  all.  Such  prunes  will  not  dry 
properly  and  will  become  " bloaters"  or  "chocolates."  Imperial 
prunes  must  be  handled  carefully,  as  the  skin  is  more  tender  than  that 
of  the  French  variety.  Some  growers  prefer  to  put  the  Imperials  out 
to  dry  for  a  few  days  before  dipping  to  avoid  injury  to  the  skin.  Like- 
wise, the  Robe  de  Sergeant  has  a  very  thin  skin  and  must  be  dipped 
but  a  very  short  time  in  a  dip  of  moderate  strength  to  prevent  entire 
removal  of  the  skin. 

After  dipping,  the  prunes  should  be  rinsed  in  clean  water.  Not 
many  dry  yards  are  equipped  to  do  this,  however.  The  prunes,  in 
some  cases,  are  then  often  shaken  down  over  a  needle  board  which  aids 
in  perforating  the  skin.  The  use  of  the  needle  board  is  especially  de- 
sirable in  certain  interior  sections  where  the  skin  is  tough.  They  are 
then  separated  into  two  sizes  and  placed  on  the  trays.  From  this  point 
they  are  taken  either  to  the  dry  yard  or  to  the  evaporaor. 

The  lengh  of  time  for  sun-drying  varies  from  about  ten  days  to 
two  weeks.  In  general,  prunes  dry  more  quickly  in  the  warm  interior 
regions  than  in  the  coast  valleys.  More  time-  is  necessary  late  in  the 
season  or  in  years  when  the  weather  is  cloudy  or  foggy.  The  large 
prunes  require  more  time  than  the  small.  Sugar  and  Imperial  prunes 
must  be  turned  often  on  the  trays  to  insure  even  drying  and  to  prevent 
molding.  Turning  is  accomplished  either  with  short  wooden  scoops 
or  with  long-handled  wooden  rakes.  No  exact  methods  are  in  use  for 
determining  when  a  prune  has  dried  sufficiently.  Some  growers  use 
one  guide  and  some  another.  When  the  prune  has  become  firm  in  tex- 
ture and  the  pit  cannot  be  separated  from  the  flesh  by  rolling  between 
the  thumb  and  first  two  fingers,  it  is  usually  considered  cured.  Another 
method  of  determining  when  the  drying  is  complete  is  by  squeezing  a 
handful.  If  the  prunes  retain  their  shape  and  do  not  mash  together, 
they  are  dried  sufficiently.  The  trays  are  then  stacked  for  a  few  days, 
after  which  the  prunes  are  placed  in  lug  boxes  and  taken  to  the  storage 
bins.  When  scraping  the  trays,  all  prunes  insufficiently  dried — known 
as  "chocolates" — are  sorted  out.  The  prunes  are  then  placed  in  stor- 
age bins  and  allowed  to  sweat  for  about  three  weeks,  after  which  they 
are  ready  for  delivery  to  the  packing  house. 


Bulletin  328  prune  GROWING  IN  CALIFORNIA  35 

For  sun-drying,  the  trays  commonly  used  are  three  by  eight  feet. 
During  an  average  season,  each  tray  is  used  usually  twice  in  the  coast 
valleys  and  possibly  three  or  four  times  in  the  interior  sections.  A 
well  filled  tray  holds  about  60  to  70  pounds  of  green  fruit.  Approxi- 
mately 1000  to  1500  trays  and  250  to  350  lug  boxes  are  necessary  for 
each  100  tons  of  green  fruit.  Approximately  one  acre  of  dry  yard  is 
considered  a  sufficient  proportion  to  20  acres  of  orchard. 

In  the  smaller  dry  yards,  at  least  five  men  are  necessary  for  eco- 
nomical dipping,  spreading,  and  placing  the  trays  in  the  dry  yard. 
One  man  dips  the  prunes,  two  men  spread  them  as  they  come  off  the 
shaker,  and  two  more  place  the  full  trays  on  the  cars  and  replace  full 
trays  on  the  receiving  platform  with  empty  ones,  After  loading  several 
cars,  four  of  the  men  take  the  trays  to  the  yard  and  distribute  them. 
The  fifth  man  attends  to  the  fire  under  the  dipping  kettle  and  moves 
up  the  full  lugs  to  a  convenient  place.  In  larger  plants,  with  fruit 
enough  to  run  continuously,  more  men  are  required,  depending  upon 
the  size  of  the  yard  and  the  track  facilities.  A  light  railway  with 
several  cars  and  one  or  more  lines  of  track,  while  expensive  to  install, 
is  convenient  for  rapid  and  economical  handling  of  the  crop. 

Evaporation  (frequently  termed  dehydration)  of  prunes  is  rapidly 
being  adopted  in  many  sections.  Many  types  of  evaporators  are  used. 
They  fall  into  two  general  classes,  the  natural  draft  and  the  forced 
draft  driers.  In  the  natural  draft,  or  stack  drier,  the  trays  are  placed 
one  above  the  other  on  cleats.  The  heat  is  applied  from  beneath  and 
is  distributed  as  uniformly  as  possible  over  the  trays  by  means  of 
baffles.  The  heating  system  may  be  either  a  steam  radiator  or  a  fur- 
nace. In  the  former,  a  series  of  coils  are  placed  beneath  the  bottom 
trays  and  the  steam  is  generated  by  a  suitable  boiler.  When  hot  air  is 
used,  the  fuel,  whether  wood  or  oil,  is  burned  in  a  furnace.  This  fur- 
nace is  often  an  old  boiler  shell.  The  hot  air  is  conducted  by  means  of 
large  iron  pipes  back  and  forth  throughout  the  length  of  the  heating 
chamber  in  order  to  secure  a  large  radiating  surface. 

The  tunnel  driers  now  being  used  are  principally  of  the  forced 
draft  type.  The  air  is  heated  either  by  steam  coils  or  hot  air  pipes  and 
is  circulated  over  the  fruit  by  means  of  a  blower  or  a  suction  fan,  which 
may  be  at  either  end  of  the  tunnel  or  in  the  middle,  according  to  the 
arrangement  of  the  heating  system.  Some  evaporators  are  equipped 
with  return  flues  which  provide  for  returning  and  reheating  part  of 
the  air  so  that  the  greatest  economy  may  be  secured.4     For  a  small 


4  For  full  technical  description  see  Calif.  Agr.  Exper.  Sta.  Bull.  No.  322,  The 
Evaporation  of  Grapes,  by  W.  V.  Cruess,  A.  W.  Christie,  and  F.  Flossfeder. 


36  UNIVERSITY   OF   CALIFORNIA — EXPERIMENT   STATION 

orchard,  a  stack  or  tunnel  drier,  as  described  in  Circular  213  of  the 
California  Experiment  Station,  would  probably  be  satisfactory. 

The  trays  used  in  evaporators  are  usually  about  3  by  3  feet,  varying 
somewhat  according  to  the  particular  make  of  evaporator.  They  are 
made  with  a  solid,  slat,  or  wire  cloth  bottom.  A  3  by  3  tray  holds 
approximately  25  pounds  of  green  prunes. 

The  length  of  time  for  drying  prunes  in  an  evaporator  varies  from 
12  to  72  hours,  according  to  the  size  of  the  prunes,  the  temperature 
maintained  and  the  type  and  efficiency  of  the  evaporator.  With  proper 
adjustment  of  temperature  and  air  humidity,  prunes  may  be  dried  very 
rapidly.  Too  high  a  temperature  near  the  end  of  the  drying  period 
causes  case  hardening  and  a  scorched  flavor.  The  temperatures  used 
vary  from  140°  to  200°  F.,  but  for  the  best  results  should  probably 
not  exceed  175°  F.  Evaporators  are  usually  so  designed  that  the  fresh 
fruit  is  started  at  the  cooler  end  of  the  evaporator.  According  to 
Professor  W.  V.  Cruess,  it  is  doubtful  if  this  is  the  best  arrangement. 

Sun-drying  and  evaporating  both  have  advantages  and  disadvan- 
tages. Sun-drying  is  usually  cheaper  and  probably  requires  less  skill. 
Large  prunes  require  several  days  longer  to  dry  than  do  the  small  ones. 

Evaporation  does  away  with  the  necessity  for  a  dry  yard.  The 
trays  may  be  used  many  times  during  the  season  instead  of  only  twice, 
as  is  the  case  in  sun-drying ;  therefore  a  smaller  number  are  required. 
The  fruit  is  not  covered  with  dust,  as  is  the  case  in  many  dry  yards. 
The  strongest  argument  in  favor  of  the  evaporator,  however,  is  the 
positive  assurance  of  curing  the  entire  crop,  regardless  of  inclement 
weather.  On  the  other  hand,  an  evaporator  involves  a  heavier  initial 
expenditure,  and  the  beginner  may  often  make  serious  mistakes  in 
evaporating  his  crop  before  he  is  familiar  with  the  process. 

COSTS  OF  PEODUCTION 

The  amount  and  kind  of  equipment  on  the  average  prune  orchard 
is  extremely  variable.  The  larger  orchards  are  usually  well  equipped 
with  all  necessary  tools  and  power.  Many  small  growers,  however,  do 
not  feel  that  they  can  afford  all  the  machinery  necessary  for  running 
their  places.  Some  prefer  to  hire  all  team  work.  Ordinarily  all 
orchards  are  equipped  with  the  common  soil-working  implements,  such 
as  plows,  harrows,  cultivators,  and  small  tools.  A  power  sprayer, 
although  important,  is  not  found  in  every  orchard.  This  fact  very 
often  works  to  the  serious  disadvantage  of  the  grower,  who  must  de- 
pend upon  loans  from  a  neighbor  or  upon  contract  spraying.  In  either 
case  it  is  often  impossible  to  make  the  application  when  it  would  do 


Bulletin  328  PRUNE  GROWING  IN  CALIFORNIA  37 

the  most  good.  In  the  same  way,  many  growers  in  sections  where 
water  is  pumped  for  irrigation  do  not  own  their  own  pumping  plant. 
If  the  lift  against  which  the  water  must  be  pumped  is  very  high,  the 
acreage  cost  of  well  and  pumping  equipment  may  be  so  large  in  small 
orchards  that  it  is  prohibitive.  The  only  alternative  is  to  buy  water 
from  a  neighbor  who  has  a  pump.  The  buyer  is  often  subjected  to 
inconvenience,  such  as  irrigating  at  night,  due  to  having  to  take  his 
turn  among  several.  Where  the  water  for  irrigation  is  obtained  from 
underground  sources,  the  cost  of  irrigation  may  be  a  large  portion  of 
the  total  cost  of  operation. 

In  addition  to  the  ordinary  orchard  machinery,  the  prune  grower 
needs  special  equipment  for  drying.  The  equipment  includes  trays, 
lug  boxes,  dipping  appliances  and  platform,  and  an  evaporator  in  case 
the  fruit  is  not  sun-dried.  '  For  a  crop  of  100  green  tons,  approxi- 
mately 1000  to  1400  or  1500  trays  are  needed.  In  the  interior  valleys 
less  may  be  needed,  but  in  the  central  coast  region,  especially  in  certain 
years,  the  larger  numbers  will  be  necessary.  Before  the  recent  in- 
creases in  cost,  these  trays  could  be  purchased  at  about  50  cents  each. 
During  the  past  few  seasons  the  cost  has  advanced  to  $1.15,  to  $1.35, 
and  in  some  cases  to  $1.50  each.  For  the  same  crop  approximately 
250  lug  boxes  at  35  cents  each  are  needed.  The  dipping  equipment 
and  the  platform  cost  from  $300  to  $500,  depending  on  kind  and  size. 

Prospective  returns  on  the  prune  orchard  cannot  be  estimated  with 
any  degree  of  certainty.  Prunes  are  grown  under  so  many  different 
climatic  conditions  and  soil  types  in  California  that  any  statement 
concerning  production  must  be  of  a  very  general  nature.  Uncontroll- 
able factors  like  those  directly  or  indirectly  influenced  by  the  weather, 
make  any  prediction  involving  exact  figures  hazardous.  Furthermore, 
no  two  growers  manage  their  orchards  alike. 

It  is  probably  safe  to  say  that  a  sustaining  crop  of  prunes  is  not 
ordinarily  produced  until  the  trees  are  six  or  seven  years  old.  For 
three  or  four  years  the  orchard  may  be  intercropped  to  provide  a 
maintenance  revenue.  During  the  fifth  and  sixth  years  the  trees 
usually  produce  enough  fruit  to  pay  expenses  and  possibly  yield  a 
small  profit.  Yields  of  mature  orchards  show  extreme  variation.  A 
fair  crop  for  central  coast  regions  may  be  estimated  at  four  or  five 
tons  of  green  fruit  per  acre.  On  fertile  soils,  with  ample  irrigation 
facilities,  the  yields  are  often  greater.  In  the  foothill  regions  the 
average  may  not  be  more  than  two  tons  per  acre.  Interior  valley 
orchards  on  deep,  fertile  soils,  particularly  on  the  so-called  river  bot- 
toms, yield  heavily,  sometimes  as  high  as  twenty  tons  green  fruit  per 
acre.    The  drying  ratio  varies,  for  different  regions,  from  2%  to  1  to 


38  UNIVERSITY   OF    CALIFORNIA EXPERIMENT    STATION 

3  to  1.  Statistics  show  that  the  average  production  of  the  state  over 
a  period  of  years  is  about  one  to  one  and  a  quarter  tons  of  dried  prunes 
per  acre. 

The  cost  ^f  all  orchard  operations  during  the  past  two  or  three 
seasons  showed  a  great  advance  over  those  of  the  preceding  years.  A 
large  part  of  this  increase  was  due  to  increased  cost  of  labor.  Costs 
of  such  operations  as  cultivating,  pruning,  spraying,  picking,  and  dry- 
ing are  directly  proportional  to  the  prevailing  wage  of  labor.  The  cost 
of  cultivation,  including  plowing,  was  probably  at  least  $20  per  acre. 
In  many  cases  it  was  more  and  in  a  few  cases  less.  Pruning  costs  vary 
somewhat  with  the  condition  of  the  trees.  Ordinarily  prune  trees  are 
not  pruned  heavily,  but  sometimes  considerable  cutting  is  necessary. 
The  average  cost  of  pruning  and  burning  brush  was  probably  not  less 
than  $15  per  acre.  Spraying  cost  approximately  $15  per  acre  per 
application.    Digging  for  borers  cost  at  least  $3  to  $5  per  acre. 

The  widest  ranges  in  cost  of  any  of  the  orchard  operations  was 
found  in  irrigation.  In  general,  irrigation  by  gravity  water  was  less 
expensive  than  irrigation  from  wells.  The  cost  probably  varied  from 
a  nominal  charge  under  some  of  the  gravity  systems  to  $35  or  $40  or 
more  per  acre  where  water  was  pumped  against  high  heads. 

Picking  cost  from  $7  to  $12  per  green  ton.  For  average  crops  of 
two  to  three  tons  of  green  fruit,  this  meant  a  cost  of  from  $14  to  $36 
per  acre.  Ordinarily  the  higher  rate  prevailed  in  orchards  having  the 
poorest  crop,  as  picking  is  done  on  a  piecework  basis.  The  drying  costs 
were  approximately  $12  per  ton,  or  from  $24  to  $36  per  acre  for  a  two 
or  three  ton  crop. 

To  the  foregoing  costs  must  be  added :  taxes,  insurance,  upkeep  of 
tillage  implements,  depreciation  of  machinery  and  buildings,  cost  of 
horse  feed,  and  interest  on  the  investment. 


STATION  PUBLICATIONS   AVAILABLE   FOR  FREE   DISTRIBUTION 


BULLETINS 


No 
185. 

251. 


253. 

261. 
262. 

263. 
266. 

267. 
268. 
270. 


271. 
272. 
273. 

274. 

275. 

276. 
278. 
279. 
280. 

282. 

283. 
285. 
286. 
288. 


Report  of  Progress  in  Cereal  Investiga- 
tions. 
Utilization  of  the  Nitrogen  and  Organic 
Matter    in    Septic    and    Imhoff    Tank 
Sludges. 

Irrigation  and  Soil  Conditions  in  the 
Sierra  Nevada  Foothills,  California. 

Melaxuma  of  the  Walnut,  "  Juglans  regia." 

Citrus  Diseases  of  Florida  and  Cuba 
Compared  with  Those  of  California. 

Size  Grades  for  Ripe  Olives. 

A  Spotting  of  Citrus  Fruits  Due  to  the 
Action  of  Oil  Liberated  from  the  Rind. 

Experiments  with  Stocks  for  Citrus. 

Growing  and  Grafting  Olive  Seedlings. 

A  Comparison  of  Annual  Cropping,  Bi- 
ennial Cropping,  and  Green  Manures 
on  the  Yield  of  Wheat. 

Feeding  Dairy  Calves  in  California. 

Commercial  Fertilizers. 

Preliminary  Report  on  Kearney  Vine- 
yard Experimental  Drain. 

The  Common  Honey  Bee  as  an  Agent  in 
Prune  Pollination. 

The  Cultivation  of  Belladonna  in  Cali- 
fornia. 

The  Pomegranate. 

Grain  Sorghums. 

Irrigation  of  Rice  in  California. 

Irrigation  of  Alfalfa  in  the  Sacramento 
Valley. 

Trials  with  California  Silage  Crops  for 
Dairy  Cows. 

The  Olive  Insects  of  California. 

The  Milch  Goat  in  California. 

Commercial  Fertilizers. 

Potash  from  Tule  and  the  Fertilizer 
Value  of  Certain  Marsh  Plants. 


No. 

290.  The    June    Drop    of    Washington    Navel 
Oranges. 

299.  The  Use  of  Lumber  on  California  Farms. 

300.  Commercial  Fertilizers. 

301.  California  State  Dairy  Cow  Competition, 

1916-18. 

302.  Control    of    Ground    Squirrels    by    the 

Fumigation  Method. 

303.  Grape  Syrup. 

304.  A  Study   on  the  Effects   of  Freezes   on 

Citrus  in  California. 

308.  I.  Fumigation   with   Liquid  Hydrocyanic 

Acid.     II.  Physical  and  Chemical  Pro- 
perties of  Liquid  Hydrocyanic  Acid. 

309.  I.  The   Carob  in   California.     II.  Nutri- 

tive Value  of  the  Carob  Bean. 

310.  Plum  Pollination. 

311.  Investigations  with  Milking  Machines. 

312.  Mariout  Barley. 

313.  Pruning  Young  Deciduous  Fruit  Trees. 

316.  The  Kaki  or  Oriental  Persimmon. 

317.  Selections  of  Stocks  in  Citrus  Propagation. 

318.  The  Effects  of  Alkali  on  Citrus  Trees. 

319.  Caprifigs  and  Caprifi cation. 

320.  Control  of  the  Coyote  in  California. 

321.  Commercial  Production  of  Grape  Syrup. 

322.  The  Evaporation  of  Grapes. 

323.  Heavy  vs.  Light  Grain  Feeding  for  Dairy 
Cows. 

Storage  of  Perishable  Fruit  at  Freezing 
Temperatures. 

Rice  Irrigation  Measurements  and  Ex- 
periments in  Sacramento  Valley,  1914- 
1919. 

Brown  Rot  of  Apricots. 

Commercial  Fertilizers. 


324. 
325. 


326. 
327. 


CIRCULARS 

No.  No. 

70.  Observations    on    the    Status    of    Corn  165. 

Growing  in  California. 

76.  Hot  Room  Callusing.  167. 

82.  The      Common     Ground      Squirrels      of  168. 

California. 

113.  Correspondence  Courses  in  Agriculture.  169. 

114.  Increasing  the  Duty  of  Water.  170. 

115.  Grafting  Vinifera  Vineyards. 

126.  Spraying  for  the  Grape  Leaf  Hopper.  172. 

127.  House  Fumigation.  173. 

128.  Insecticide  Formulas.  174. 

129.  The  Control  of  Citrus  Insects.  175. 

130.  Cabbage  Growing  in  California. 

131.  Spraying  for  Control  of  Walnut  Aphis.  176. 
135.  Official  Tests  of  Dairy  Cows. 

137.  Wood  Decay  in  Orchard  Trees.  177. 

138.  The  Silo  in  California  Agriculture.  178. 

139.  The  Generation  of  Hydrocyanic  Acid  Gas  179. 

in  Fumigation  by  Portable  Machines. 

143.  Control    of     Grasshoppers    in     Imperial  181. 

Valley.  182. 

144.  Oidium  or  Powdery  Mildew  of  the  Vine. 

148.  "Lungworms".  183. 

152.  Some  Observations  on  the  Bulk  Handling  184. 

of  Grain  in  California.  185. 

153.  Announcement    of    the    California    State 

Dairy  Cow  Competition,  1916-18.  188. 

154.  Irrigation    Practice    in    Growing    Small  189. 

Fruits  in  California.  190. 

155.  Bovine  Tuberculosis.  191. 

156.  How  to  Operate  an  Incubator.  193. 

157.  Control  of  the  Pear  Scab.  198. 

158.  Home  and  Farm  Canning.  201. 

159.  Agriculture  in  the  Imperial  Valley.  202. 

160.  Lettuce  Growing  in  California. 
164.  Small  Fruit  Culture  in  California. 


Fundamentals    of    Sugar    Beet    Culture 

under  California  Conditions. 
Feeding  Stuffs  of  Minor  Importance. 
Spraying  for  the  Control  of  Wild  Morning- 

Glory  within  the  Fog  Belt. 
The  1918  Grain  Crop. 
Fertilizing  California  Soils  for  the   1918 

Crop. 
Wheat  Culture. 

The  Construction  of  the  Wood-Hoop  Silo. 
Farm  Drainage  Methods. 
Progress  Report  on  the   Marketing  and 

Distribution  of  Milk. 
Hog  Cholera  Prevention  and  the  Serum 

Treatment. 
Grain  Sorghums. 

The  Packing  of  Apples  in  California. 
Factors  of  Importance  in  Producing  Milk 

of  Low  Bacterial  Count. 
Control  of  the  California  Ground  Squirrel. 
Extending  the  Area  of  Irrigated  Wheat  in 

California  for  1918. 
Infectious  Abortion  in  Cows. 
A  Flock  of  Sheep  on  the  Farm. 
Beekeeping  for  the  Fruit-grower  and  Small 

Rancher  or  Amateur. 
Lambing  Sheds. 
Winter  Forage  Crops. 
Agriculture  Clubs  in  California. 
Pruning  the  Seedless  Grapes. 
A  Study  of  Farm  Labor  in  California. 
Syrup  from  Sweet  Sorghum. 
Helpful  Hints  to  Hog  Raisers. 
County     Organization     for     Rural     Fire 

Control, 


CIRCULARS — Continued 


No. 

203.  Peat  as  a  Manure  Substitute. 

205.  Blackleg. 

206.  Jack  Cheese. 

207.  Neufchatel  Cheese. 

208.  Summary  of  the  Annual  Reports  of  the 

Farm  Advisors  of  California. 
210.  Suggestions  to  the  Settler  in  California. 

214.  Seed    Treatment   for   the    Prevention    of 

Cereal  Smuts. 

215.  Feeding  Dairy  Cows  in  California. 

217.  Methods    for    Marketing    Vegetables    in 

California. 

218.  Advanced  Registry  Testing  of  Dairy  Cows. 


No. 

219. 
220. 
221. 


The  Present  Status  of  Alkali. 

Unfermented  Fruit  Juices. 

How  California  is  Helping    People  Own 

Farms  and  Rural  Homes. 
Fundamental  Principles  of  Co-operation 

in  Agriculture. 
The  Pear  Thrips. 
224.   Control  of  the  Brown  Apricot  Scale  and 
the  Italian  Pear  Scale  on  Deciduous 
Fruit  Trees. 
Propagation  of  Vines. 
Protection    of    Vineyards    from    Phyl- 
loxera. 


222. 
223. 


225. 
226. 


